1. Field of the Invention
The present invention relates to protein-containing aqueous solutions, methods for increasing the protein concentration of aqueous solutions and a protein preparation and to techniques applicable in the preparation of pharmaceuticals for clinical use using physiologically active proteins.
2. Description of the Prior Art
In using a protein as a homogeneous component, it is extremely important to dissolve the protein in a solvent. For example, where a certain amount of a protein is fractionated from a composition which contains that protein or where analysis of the protein is made, the composition containing the protein must be homogeneous. Furthermore, when a protein is dissolved in water for administration as a pharmaceutical such as an injectable preparation, the protein has to be completely dissolved.
In general, the solubility of proteins in aqueous solvents is strongly affected by hydrophilic or hydrophobic residues present on the surface of the protein and by charges on the protein. When the protein is only slightly dissolved because of the presence of hydrophobic residues on the surface of the protein, it is possible to increase the solubility by adding a surfactant.
On the other hand, when the pH of an aquaeous solvent is near the isoelectric point of the protein to be dissolved, which readily causes isoelectric precipitation, solubility of the protein can be increased by Increasing the salt concentration and the ionic strength of the aqueous solvent. In this case, a surfactant does not contribute to the increase of the protein solubility. Furthermore, when the pH of an aqueous solvent is near the isoelectric point of a protein and the salt concentration is low, the protein is soluble only at a relatively low concentration. Therefore, in order to dissolve the protein in a relatively high concentration, either a method in which a pH separate from the isoelectric point is used or a method in which the salt concentration is increased is generally used.
However, in some cases, It Is necessary to dissolve a protein at a sufficiently high concentration without increasing the salt concentration at the pH near the isoelectric point. An example is when a physiologically active protein having an isoelectric point near neutral pH is to be administered, in a form of a solution at a pH near neutral, to a patient who should maintain his or her salt Intake as low as possible. In this case, the only possible technique has been either to use the protein in a lower concentration or to inevitably administer salt, which is a serious practical problem to be solved in the formulation of protein active pharmaceuticals.